Compilare: user.cpp: In function ‘int main()’: user.cpp:59:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (i=0;i<b.size();i++) ^ user.cpp:62:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (i=0;i<c.size();i++) ^ user.cpp:65:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (i=0;i<d.size();i++) ^